Thinking makes it so

Reading through some texts, I found an interesting passage. It appears in a play by an author named William Shakespeare. The quote reads: "There is nothing good or bad, but thinking makes it so". This sounded familiar to me, and with good reason. A philosopher closer to my time named Seneca said the same thing. But what does it mean? It sounds suspiciously relativist and reminds me of a similar claim made by Protagoras, who lived during my time in Athens. He said "Man is the measure of all things", meaning that truth or falsity is dependent upon one's subjective point of view. Now, Seneca was not talking about objective truth. He was talking about moral action and values. And I agree with his statement, in a sense, but I do not agree with relativism. So I worry that there is tension in my beliefs. Consider this syllogism:

P1. (premise) If there is no objective good or bad, then good or bad is based on subjective thought

P2. (premise) There is no objective good or bad (Seneca)

C. (conclusion) Therefore, good or bad is based on subjective thought

But I think the conclusion is false. I have argued many times that moral good or bad is not subjective, which means it is objective:

P1. (premise) If there is no objective good or bad, then good or bad is based on subjective thought

P2. (premise) It is not true that good or bad is based on subjective thought

C. (conclusion) Therefore, there is objective good or bad.

So, I wonder what Seneca meant. How can I reconcile my belief that there is objective good and bad with his claim that "thinking makes it so"? Perhaps he was merely talking about the feelings we have towards certain events. An event may be objectively good or bad, but my feeling about the event is subjective. It is up to me to respond to the event. So I believe he meant that whether we feel angry or upset about an event is the result of our thinking about things.

-- Socrates.

** Update:
To answer a question that was posed "How do I know that the world is divided into objective and subjective things", I answer that I do not know. I know very little. But here I am analyzing the relativist's claim that there is no objective good and that good and bad are subjective. I am testing my beliefs against this claim. My beliefs are not yet knowledge.
- S

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